1. Field of the Invention
The present invention relates to a multibit ΔΣ modulation DA converter.
2. Description of the Related Art
A multibit ΔΣ modulation DA converter is generally known as a DA converter that converts a multibit digital signal such as a PCM (Pulse Code Modulation) signal to an analog signal. See for example FIG. 13 of Japanese Patent Application Laid-Open Publication No. 2002-368620. Such a DA converter is provided with a plurality of internal DA converters, and signals output from a multibit ΔΣ modulator are sorted by a dynamic element matching circuit (hereinafter, referred to as a DEM circuit) into the internal DA converters.
When performing the multibit ΔΣ modulation in this way, it is known that a noise called idle tone is generated by DC offsets included in the multibit digital signals, due to variations of element levels. Therefore, various methods have been used for preventing the generation of the idle tone.
For example, FIG. 13 shows a conventional example of a configuration of a DA converter that prevents the generation of the idle tone with the use of a zero-point correction circuit. In a DA converter 200 of FIG. 13, the input PCM signal is enhanced in the sampling rate by an interpolation filter 201 and is noise-shaped by a multibit ΔΣ modulator 202. The output signal of the multibit ΔΣ modulator 202 is split by a DEM circuit into a plurality of signals, which is input into a plurality of multibit internal DA converters (hereinafter, referred to as MDACs) 204. An adder 205 adds up the signals output from a plurality of the MDACs 204 to acquire a signal that is an analog signal converted from the input PCM signal.
The DA converter 200 is provided with a zero-point correction circuit 206 that outputs a DC offset opposed to the DC offset included in the PCM signal. Since the DC offset output from the zero-point correction circuit 206 is added to the PCM signal by the adder 207, the DC offset included in the PCM signal is canceled and the generation of the idle tone is constrained.
FIG. 14 shows a conventional example of a configuration of a DA converter that prevents the generation of the idle tone with the use of a dither circuit. A DA converter 210 of FIG. 14 is provided with a dither circuit 211 that outputs a dither signal, instead of the zero-point correction circuit 206 of FIG. 13. In such a configuration, the generation of the idle tone can be constrained by applying a dither signal large enough to mask the DC offset included in the PCM signal.
FIG. 15 shows a conventional example of a configuration of a DA converter that prevents the generation of the idle tone with the use of a high-pass filter. A DA converter 220 of FIG. 15 is provided with a high-pass filter 221, instead of the zero-point correction circuit 206 and the adder 207 of FIG. 13. In such a configuration, the generation of the idle tone can be prevented by removing the DC offset included in the PCM signal with the high-pass filter 221.
However, in the case of the method using the zero-point correction circuit described above, a circuit scale of the zero-point correction circuit is large, which causes cost increase.
In the case of the method using the dither circuit, while a circuit scale can be reduced as compared to the case of using the zero-point correction circuit, if the dither signal is enhanced to be able to mask the DC offset included in the PCM signal, the dither signal itself may generate harmful effects. Therefore, to avoid such harmful effects, complicated circuit design, etc. are needed, which has been technically difficult.
While the method using the high-pass filter has a small circuit scale and is technically easy, it is problematic that a sound quality is deteriorated because the high-pass filter is added to a signal path. The cutoff frequency of the high-pass filter is, for example, on the order of 1 Hz, which is sufficiently low as compared to the audible band, and it is theoretically believed that the audible band is not affected. However, as a result of experiments, more people feel that the volume of low pitch sound is louder and the sound is more real when the high-pass filter is not added.